1. Field of the Invention
The present invention relates to optical disc write and/or read (hereinafter write/read) apparatus which rotate edge-constrained discs and more specifically to improve disc-turntable interface constructions of such apparatus.
2. Brief Description of Prior Art
U.S. Pat. No. 4,365,258 issued Dec. 21, 1982 to Geyer and Leonard, discloses configurations for optical disc units and cooperative write/read apparatus that provide significant advantages in using flexible optical discs. In one disclosed disc unit configuration a flexible record disc and a flexible disc cover are engaged around annular peripheral regions to uniformly tension the disc and its cover in spaced relation. In some embodiments the tension is sufficient to provide a relatively smooth and flat disposition of the record disc and the peripheral support structure of the disc unit cooperates with turntable structure of the write/read apparatus to accurately position the record disc with respect to the apparatus write/read lens. In other embodiments the apparatus turntable comprises an annular abutment surface which contacts an annular region of a mounted record disc to accurately position the record disc with respect to the write/read lens.
In both of the above embodiments it is preferable to have a space between at least the record zone portion (i.e. the annular portion between the outer periphery and center where write/read occurs) of the record disc and adjacent turntable portions. That is, it is advantageous not to have contact between any record zone portion of the record disc and opposing turntable portions because contact could cause disruptions in the smooth and flat disposition that is desired.
The above described disc-unit/apparatus configurations perform admirably. However, there are certain features of these systems where improvements might be desired for some applications. For example, during rotation an air pressure variation develops radially within the sealed disc unit and within the enclosed space which exists between the record disc and turntable portions that are radially interior of the edge support or abutment surfaces. That is, centrifugal force on the air in these enclosed spaces causes relatively higher pressure (e.g. above ambient) at radially exterior zones and relatively lower pressure (e.g. sub-ambient) at radially interior zones. Without air leakage from either air region, such radial pressure variations are predictable for a given configuration, and are generally equal on opposite sides of the record disc. The flexible record disc and the disc cover experience limited deflections according to the radial pressure distribution within the rotating sealed disc unit (bending slightly above its stationary position at outer radii and slightly below its stationary position at inner radii).
Such predictably limited deflections can be accommodated by the acquisition range of the write/read lens. However, if air leakage occurs at some portion of the disc-turntable interface, the radial pressure distribution within the enclosed region between the turntable and the record disc can differ more drastically from that within the rotating sealed disc unit. Accommodation of the resulting record disc deflections becomes much more difficult. Such deflections are not predictable because they depend on the location and extent of the leakage. The leakage may occur gradually or suddenly during write/read operation. Gradual leakage at best extends the required acquisition range of the write/read lens. Sudden changes of the record disc position can cause inaccuracy in the write/read operation. Moreover, since the leakage-caused deflections are transmitted to the disc-unit cover sheet, a sudden leakage can cause a crash between the cover sheet and the write/read lens.
By careful tolerancing of the disc-unit turntable interface structure (e.g. the peripheral support structure, the annular abutment surface and the center hub structure, if used), the problems of air leakages can be avoided. However, it is desirable, particularly with respect to fabrication of the disc unit, that highly precise tolerances not be required.